The present invention relates to the production of photographs and, more particularly, to the control of the contrast in those photographs.
In the course of producing photographs directly with a camera, through slide duplication or through enlargement, there can be problems with contrast. Typically, the contrast increases during normal printing and duplicating processes. This problem is particularly significant when making prints or duplicates of 35 mm slide transparencies, but is also present in other photographic processes.
A well known method of reducing contrast is through controlled fogging of the film, i.e. exposure of the film to controlled and small amounts of diffuse light. This may be done before, during or after the imaging exposure of the film.
One means of automatically fogging film is shown in FIGS. 1 and 2 of U.S. Pat. No. 4,021,831 of Bercher. In this device, light from an object is collected by a lens and focused on film. Located between the lens and the film is a light conductor which directs light from a local source obliquely onto the film.
A disadvantage of this arrangement is that the distribution of the fogging light from the fiber optic cable cannot be well controlled at different magnifications. Therefore the contrast reduction will vary from place to place on the film. A second disadvantage is that the camera must be modified to accept the fiber optic cable. Consequently, the user is not free to use any camera he chooses.
An alternative way of controlling contrast is illustrated in FIG. 3 of the Bercher patent. In this arrangement a semi-transparent mirror is interposed at a 45.degree. angle between the lens and the film. The light image passes through the mirror to the film, and diffuse light from a local source is directed at a right angle to the image light path on to the mirror. The mirror reflects it in line with the image light and onto the film.
Even though the imaging light and the fogging light are in the same general direction in this arrangement, they are not colinear. The fogging light is spread out over a much wider area and is difficult to control at different magnifications.
Both of the arrangements in the Bercher patent require a second local light source in the diffusion control device. This not only increases the complexity and cost of the system, it requires a means for synchronizing the turning on and off of the two light sources. Further, any changes in the intensity or color of the primary light source due to lamp aging or the use of color filters must be compensated manually at the contrast control lamp by adjusting the intensity and using matching color filters. Also both methods require modification to the camera.
A further device is shown in FIG. 4 of the Bercher patent. It uses the light which creates the image to produce the fogging light and is also positioned outside the camera so the camera need not be modified. However, like the other devices its fogging light is not in the form of a cone of light approximating the cone of imaging light. Thus the contrast is not well controlled over the entire negative in any of these prior devices.